290 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 12 



in attack by the biological method because if we can liberate a surplus 

 of natural enemies of the pest when they are normally at low ebb, 

 the host also being scarce we should be able to prevent the host insect 

 from gaining that ascendency which makes it sooner or later a pest. 

 Under these conditions many of the natural enemies would, of course, 

 die from starvation and the premises would have to be restocked from 

 time to time. 



From a biological standpoint the proposition seems a sound one, 

 but what entomologists and agriculturists want to know is whether 

 or not it can be put into practice. The writer believes that there are 

 fundamental principles involved in this work which in the case of 

 certain pests would so limit the possibilities that an attempt to apply 

 it to field conditions would not be justifiable, while in the case of 

 other pests, or under other conditions, the opposite would be true. 

 In other words, a decision as to its applicability in the control of any 

 pest should be arrived at only after a careful study of each individual 

 case with reference to the limiting factors. 



Among the factors which govern the possibilities in this direction 

 the following are important : 



1. Comparative Reproductive Capacity or Host and Avail- 

 able Entomophagous Insects. This is obviously of much impor- 

 tance. If the host insect breeds with great rapidity and the only 

 available entomophagous insects are of low reproductive capacity, 

 success in the attempt could hardly be expected. 



2. Power of Locomotion of Pest and Natural Enemies. As a 

 general rule attempts to control an insect pest by the biological method 

 will be undertaken locally, and not over the entire range of the insect. 

 If the pest is an active flier, its ability to continually and quickly 

 reinfest the area where the attempt is being made would react unfav- 

 orably on the effort. If the entomophagous insects are active fliers 

 it is possible that their tendency to disperse rapidly from the place 

 of liberation may also work against the success of the undertaking. 



3. Sequence of Available Entomophagous Insects. It is very 

 desirable, especially where use is being made of parasitic rather than 

 predaceous insects, that there be a complete sequence of parasites 

 affecting the egg, larva and pupa of the pest. This is for the reason 

 that multiple parasitism, or parasitism by two or more different species 

 in the same individual of the host insect, at times reduces the efficiency 

 of the parasites as a whole. The ideal arrangement would be a single 

 efficient parasite for each of the three stages of the host, but this is not 

 a necessity, especially if predaceous insects are also employed. 



4. Possibility of Rearing or Obtaining the Entomophagous 

 Insects in Sufficient Quantities. This is one of the most impor- 



